Expansible work holder

ABSTRACT

An expansible work holder including a deflectable shell, an expansible insert lining the interior of the shell, a single fluid chamber defined at least in part by the insert, and means for establishing fluidic pressure within said chamber. The pressure establishing means may include a device for adjusting expansion of the holder prior to insertion of the holder into the workpiece and a device for expanding the holder after such insertion. The shell may have selected portions of a different size than the remainder of the shell for aiding the force fit of the holder within the workpiece.

United States Patent 1191 Piotrowski et al.

1451 July 24, 1973 1 EXPANSIBLE WORK HOLDER 3,497,226 2/1970 Hohwart et a]... 279/2 2,971,765 2/1961 Atherholt 242172 B [761 Inventms S S92 2 22 ff f 3,225,418 12/1965 Fara 242/72 B ear orn, 1c au 3:3 gz Berkley Primary ExaminerRobert L. Spruill' Attorney-Hauke, Gifford and Patalidis [22] Filed: May 6, 1971 211 Appl. No.: 140,815 [571 ABSTRACT An expansible work holder including a deflectable shell, an expansible insert lining the interior of the gl M 82/44 325 shell, a single fluid chamber defined at least in part by [58] Fie'ld I20 48 l the insert, and means for establishing fluidic pressure 279/2 i within said chamber. The pressure establishing means may include a device for adjusting expansion of the holder prior to insertion of the holder into the work- [56] References Cited p1ece and a device for expandmg the holder after such UNITED STATES PATENTS insertion. The shell may have selected portions of a dif- 2,318,838 5/1943 Conradson 82/44 ferent size than the remainder of the shell for aiding the fil i et force fit of the holder within the workpiece. mg er 3,025,737 3/1962 Hohwart et al.... 28 Claims, 6 Drawing Figures 38 o l4 l6 I8 36 34 I8 \\\\\W\\\ U I l I\ \ll\ PAINTED- SHEEI 2 OF 2 Eiiiii. FIG.4

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INVENTORS PAUL T. WAITE BY CHARLES PIOTROWSKI W M,WA

ATTORNEYS EXPANSIBLE WORK HOLDER BACKGROUND OF THE INVENTION I. Field of the Invention This invention relates to fluidic expansible arbors, mandrels or work holders for accurately locating and holding external or internal workpieces or tool elements for machining, grinding, gauging and other operations.

II. Description of the Prior Art l-leretofore, conventional work holders utilized double walled cylinders or shells, the outer of which was forced to expand through the medium of sealed internal fluidic pressure. If the work holder was inserted into a hole in a workpiece, expansion of the shell resulted in a tight fit of the holder or mandrel into the workpiece. These devices included a first chamber or reservoir for the fluid, and a second chamber connected by a narrow passageway to the first chamber, into which the fluid was forced under pressure to expand the work holder.

Various problems may be encountered with these prior art mandrels and it was believed that placing slots in the outer shell would facilitate radial expansion. Experimentation has resulted in slots parallel to, perpendicular to and diagonal with respect to the axis of the mandrel.

Another suggestion for improvement was based on the relative thickness of the walls of the inner and outer cylinder and various combinations of thin and thick cylinder walls have been used.

A serious problem with double walled cylinders is the sealing of the fluid between the support member or body portion and the expansible shell since radial expansion of the shell tends to separate it from the support member.

Other mandrels shown in the prior art include a tapered body portion to facilitate insertion of the holder into the workpiece. This was believed necessary because of the limited degree of expansion available with double walled mandrels. When thin-walled mandrels are used to permit easier expansion, the thin walls may rupture under the great pressure established by the fluidic system.

Another mandrel shown in the prior art is made by expanding the arbor and grinding it to a true cylinder while so expanded to its work holding position, so that, theoretically, the area of engagement between the workpiece and the arbor when in use would be greater than with a mandrel which is cylindrical when in the non-expanded condition since the latter has substantially a single line contact when expanded because the outer shell or sleeve tends to bulge" in the center on expansion. Obviously, such a true cylinder" configuration produces this greater contact area only for one diameter hole in a workpiece; any other hole diameter results in the same poor fit between the workpiece and arbor experienced using conventional arbors.

One problem with the mandrels heretofore known is the failure to consider the true effects of fluid pressures transferring between the inner and outer chambers of the double-walled mandrel which tends to stress the ,intermediate wall and eventually produce cracks and fluid leakage.

SUMMARY OF THE INVENTION Accordingly, with these problems in mind, the present invention contemplates a solution by providing a new and improved expansible mandrel having only one fluidic chamber.

It is an object of the present invention to provide an expansible mandrel or arbor which does not subject the internal structure to extreme pressures.

It is yet another object of the present invention to provide an expansible holding device which is able to forcibly expand within workpieces of different diameters and still provide a satisfactory fit.

These and other objects are accomplished in an expansible work holder including a support member, an expansible outer shell, an insert for said shell, means cooperating with said insert to define a single fluid chamber and means for establishing fluidic pressure within said chamber. The pressure establishing means may include actuating and adjusting means and the shell may have certain portions of a different size than the remainder of the shell for providing a better fit within the workpiece.

DESCRIPTION OF THE DRAWINGS The foregoing objects of the present invention, together with other objects and advantages which may be attained by its use, will become apparent upon reading the following detailed description in conjunction with the accompanying drawings, wherein like reference characters refer to like parts throughout the several views, and in which:

FIG. 1 is a longitudinal cross-sectional view of a preferred expansible holding device embodying the present invention;

FIG. 2 is a longitudinal cross-sectional view of another embodiment of the invention illustrating radial workpiece-engaging lands;

FIG. 3 is a longitudinal cross-sectional view of a further modification of the invention illustrating separate adjusting and actuating means;

FIG. 4 is a cross-sectional view illustrating an optional element which may be used in the structure of the present invention for filling the chamber with fluid;

FIG. 5 is a fragmentary cross-sectional view illustrating another adjusting means of the present invention; and

FIG. 6 is a longitudinal cross-sectional view of a combined adjusting and actuating means constructed according to the principles of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Referring first to FIG. 1 of the drawings, the expansible mandrel or work holder includes a support member 10 having a longitudinal bore 12. One end of the .support member (the right end as seen) includes a reduced-diameter portion 14.

Mating with support member 10 and enclosing the portion 14 is a cylindrical shell 16. The shell may be made of an expansible material, such as steel, and may have at least one slot 18; preferably extended :helically around the shell, to facilitate radial expansion. In the preferred embodiment of FIG. 1, the shell 16 is generally U-shaped in longitudinal cross-section.

Within the shell 16 is an insert or liner 20. The insert may be made of a material which is relatively more flexible than the shell 16. For example, an insert of nylon may be used. The insert 20 is adapted to completely cover the interior of the shell 16, thus in the illustration of FIG. 1 the insert 20 is also generally U- shaped in longitudinal cross-section. Insert 20 includes a flange portion 22 and, when the insert 20 is fitted within the shell 16 and the shell 16 mated with the support member 10, the flange portion 22 of the insert 20 abuts the end of the portion 14 of the support member 10.

The work holder includes pressure producing means which will now be described. A plunger 24 is longitudinally extendable from one end of the bore 12, and the opposite end of the bore 12 (left end as seen) has an internal threaded portion 26. A set screw 28 is threaded into the bore of body member 10. The plunger is movable longitudinally along the axis of the support member by advancing or withdrawing the set screw 28.

Cup seals 30 surround the plunger 24 and fit within the flange 22 of the insert 20. The insert and the end of the plunger 24 define a single sealed fluid chamber 32 within the insert 20, and on the opposite side of the insert 20 from the shell 16.

In preparing the work holder for use, the set screw 28 is withdrawn and the plunger 24 is removed. The reservoir or chamber 32 is then filled with a substantially incompressible liquid or fluid, such as oil or heavy grease. The plunger 24 is then inserted through the bore 12 of body member 10 past the cup seals 30 to close the fluidic chamber 32. The set screw 28 may now be threaded into the bore 12 of the body member 10 and advanced into contact with the plunger 24.

In operation, the work holder is inserted into a hole within the workpiece shell end 16 first. Care must be taken not to insert the work holder too far into the workpiece. Once the work holder is properly inserted, the set screw 28 may be advanced. This forces the plunger 24 further into the chamber 32 and increases fluidic pressure therein. The increase in pressure causes the insert 20 to expand radially, exerting pressure against the inner wall of the shell 16 and causing it to expand radially. The purpose of the optional slots 18 is to facilitate this radial expansion of the shell 20. The further the set screw is advanced, the greater penetration is effected by plunger 24, increasing further the fluidic pressure within chamber 32 and thus causing a greater expansion of the insert 20 and the outer shell 16. When the shell 16 expands into holding contact with the workpiece, further advancing of the set screw 28 is no longer necessary. With the work holder in holding contact, the workpiece may be subjected to the desired machining operations such as by inserting the support member 10 into the chuck of a lathe.

After machining or other work operations are completed, the support member 10 may be removed from the lathe and the set screw 28 may be loosened, so that the shell 16 may be removed from the workpiece.

It will be noted that in the present invention, basically, the work holder includes only a single fluid chamber or reservoir 32 and that there is no'excess strain on the insert 20 because there is no fluid flow through or around the insert, but only pressure fluid on one side of the insert.

As previously noted, various attempts have been made to provide better gripping or holding forces between the expansible work holder and the workpiece, i.e. a better force fit. With reference to the shell 16 of FIG. 1, it may be seen that a portion 34 is of a different size than the remainder of the shell. As shown, portion 34 is smaller or narrower than the remainder of the shell 16. When the shell expands in a workpiece, the regions or points 36, 38 immediately adjacent to the narrower portion 34 will both come into contact with the interior wall of the opening in the workpiece. Thus,

two lines 36, 38 (around the circumference of shell 16) will contact the workpiece, resulting in a twopoint fit. Such a fit reduces slippage between the holder and the workpiece which may otherwise result from a one-point" fit effected by the bulging of a single diameter cylindrical sleeve.

In lieu of the narrower area 34 of FIG. 1, two or more outward radial projections or lands 40, 42 may be provided on the shell 16 as illustrated in FIG. 2. Thus, portions of the shell 16 are of a different size, i.e. larger than the remainder of the shell, and, on expansion of the shell 16, a two-point contact is similarly effected at the projections 40, 42, resulting in the better fit between the work holder and the workpiece.

FIG. 3 illustrates additional features of the present invention which may be employed. It is noted that the shell 16 in this modification is formed as a generally hollow cylinder having a larger internal diameter for one part of its length. That portion having the smaller internal diameter has an internal threaded portion 26' corresponding to the threaded portion 26 of the support member 10, but located at the opposite end of the work holder. The insert 20 is formed as a sleeve or hollow cylinder and has flanges 22, 22' at each end. Additional cup seals 30 are provided at the right or shell" end of the insert 20, i.e. the end of the insert opposite from the support member 10. These cup seals 30 are similar to the cup seals 30.

While the support member 10 is again provided with a plunger 24 and its associated set screw 28, the shell 16 is also provided with a plunger 24' and an associated set screw 28. The insert 20 and the inner ends of the plungers 24, 24 define a single sealed fluidic chamber. The set screw 28' and the plunger 24' are used to provide a gross initial adjustment for the work holder. Such a gross adjustment might be used, for example, prior to insertion of the holder into the workpiece. Thus, advancing the set screw 28 advances the plunger 24' to the left (towards the reservoir or chamber 32) to establish or increase fluidic pressure and initiate a degree of expansion of the insert 20 and shell 16. Once this gross adjustment is completed and the work holder is inserted into the workpiece, the set screw 28 is advanced to force the plunger 24 into the chamber 32 as previously explained. This further increases the fluidic pressure, causing additional expansion of the shell 16 to create a tight fit between the shell 16 and the workpiece. Thus, the embodiment of FIG. 3 also illustrates a single fluid chamber expansible mandrel which is operable according to the principles of the present invention.

As illustrated in F IG. 3, the previously described optional slots 18 may be provided in the shell 16. Similarly, a narrower region 34 (as in FIG. 1) or lands 40, 42 (as in FIG. 2) may be provided with this embodiment of the invention.

It is convenient in some cases to provide a specific means for refilling the chamber 32 with fluid. FIG. 4, for example, illustrates a plunger 24", similar to the plunger 24 of FIG. 3, provided with an internal threaded recess 43 into which is loosely threaded a set screw 44 spaced slightly from a ball check 46 disposed in the end of the recess 43 and normally closing the inner end of a passage 48. To fill the reservoir, the set screw 28' holding the plunger in the sleeve 16 (FIG. 3) is removed and fluid is injected into the threaded portion 26', forcing the ball check 46 open and working past the threads of the set screw 44 into the chamber 32. Pressure in the chamber 32 will normally hold the ball check 46 seated over the passage 48.

FIG. 5 illustrates yet another device for establishing fluidic pressure within a single chamber 32. The plunger, such as the plunger 24 of FIG. 1, has been omitted entirely and a set screw 50 is threaded through a bore 51 in the shell 16. The set screw is disposed in direct contact with the insert 20. Advancing the set screw 50 displaces the end of the insert 20 inwardly, thus compressing the fluid within the chamber 32 and creating the desired fluidic pressure which will cause expansion of the insert 20 and consequent expansion of the shell 16.

The set screw of FIG. 5 may be used only to replace the set screw 28, plunger 24' and cup seals 30' of FIG. 3, if desired, or a completely sealed unit could be made including a hollow fully closed insert filled with the relatively incompressible fluid. The insert would be of a size to fit closely within a shell, such as shell 16, and the shell could have threaded bores 51 at each end to accept set screws 50, such as illustrated in FIG. 5. Gross adjustment would then be effected by advancing one set screw 50, and final holding expansion pressure would be effected by advancing the other set screw 50. The device illustrated in FIG. 5 may be used to replace either the gross adjustment means or the fine actuation means of FIG. 3.

FIG. 6 illustrates yet another embodiment of the invention in which the mandrel, similar to that of FIG. 1, includes a support member having a bore 12, a shell 16 which may be slotted, anins ert 20, and cup seals 30. A first plunger 52 having a hollow bore 54 is disposed within the bore 12 as in FIGS. 1, 2 or 3, and a second plunger 56 is disposed within the bore 54 of the first plunger 52. Cup seals 58 provide a seal between the inner second plunger 56 and an enlarged portion of the bore 54 of the outer first plunger 52.

Each plunger has its associated set screw for longitudinally advancing the plungers. A set screw 60 is threaded within a larger set screw 62, respectively abutting the outer ends of the plungers 56, 52. The set screw 62 has an interior threaded element 64 which acts to retain the cup seals 58 in place and provide a seat for the innerset screw 60.

The single sealed fluidic chamber 32 is thus defined by the insert and the inner ends of the plungers 52 and 56.

When it is desired to provide gross adjustment of the mandrel, the set screw 62 is advanced, thus moving both plungers 56 and 52 together into the chamber 32. The mandrel may then be inserted into the workpiece, as previously explained, and final expansion of shell 16 may be effected by advancing the inner set screw 60, which moves only the inner plunger 56, inward to effect the necessary increased pressure within the chamber 32.

In each of the illustrated embodiments it will be clearly understood that the optional lands or projections of HO. 2 or the narrower area of FIG. 1 may be included. Each embodiment of the present invention includes the principle of a single sealed fluidic chamber with the insert fitting tightly within the shell. Where various portions of the present invention have been referred to as right" or left, it will be appreciated that this is done merely for the purpose of illustration and facilitating description; also it should be understood that many changes and modifications may be made without departing from the spirit of the invention or the scope of the appended claims.

What is claimed is:

1. An expansible work holder comprising: a support member having a longitudinal bore open to an end of said member,

a tubular radially deflectable shell extending from said end of said support member and having a closed end to define a chamber wholly within said shell between its closed end and said end of said support member,

an expansible liner closely fitted within and substantially filling said chamber and having a fluid-filled recess opening to and registering with said bore at said end of said support member,

a plunger extending from said bore into said reces and means selectively actuating said plunger to vary the degree of extension thereof into said recess and thereby vary the fluid pressure therein acting expansively on said liner and consequently on said shell.

2. The holder of claim 1 wherein said plunger fits closely on its diameter within said liner recess.

3. The holder of claim 1 wherein said bore has a threaded outer end and said actuating means comprises a set screw adjustably carried in said outer end and bearing on the end of said plunger in said bore.

4. The holder of claim 1 and including a seal element surrounding said plunger adjacent the opening of said recess to said bore for sealing against fluid leakage from said recess.

5. The holder of claim 1 wherein the closed end of said shell has a longitudinal bore therethrough and the recess in said liner extends therethrough to be open to and register with said shell bore, and including a second plunger extending from said shell bore into said recess, and means selectively adjusting said second plunger to vary the degree of extension thereof into said recess and thereby initially adjust the fluid pressure therein which may be subsequently varied by actuation of said first plunger.

6. The holder of claim 5 wherein said second plunger fits closely on its diameter within said liner recess.

7. The holder of claim 5 wherein said shell bore has a threaded outer end and said adjusting means comprises a set screw adjustably carried in said shell bore outer end and bearing on the end of said second plunger in-said shell bore.

8. The holder of claim 5 and including a seal element surrounding said second plunger adjacent the opening of said recess to said shell bore for sealing against fluid leakage from said recess.

9. The holder of claim 1 wherein said plunger has a bore therethrough open to said recess and a second plunger longitudinally movable therein to adjust fluid pressure in said recess.

10. The holder of claim 9 and including a seal element surrounding said second plunger and sealing same within said first plunger bore against fluid leakage from said recess.

11. The holder of claim wherein said second plunger has a bore therethrough provided with a oneway valve to provide for selectively filling said recess with fluid.

12. The holder of claim 1 wherein said shell includes a portion of a different size than the remainder of said shell for providing a two-line contact with a bore within a workpiece upon expansion of said holder and consequently said shell.

13. The holder of claim 12 wherein said portion of a different diameter comprises a slightly smaller diameter region of said tubular shell.

14. The holder of claim 12 wherein said portion of a different diameter comprises a pair of longitudinally spaced annular lands.

15. The holder of claim 1 and including means selectively displacing a portion of said liner inwardly to reduce the volume of said recess for adjusting the initial fluid pressure therein which may be subsequently varied by actuation of said plunger.

16. The holder of claim 1 wherein said shell is separable from and removably secured to said support member.

17. The holder of claim 1 wherein said liner recess is of uniform diameter'throughout its length.

18. The holder of claim 1 wherein said tubular member bore from which the plunger extends and said liner recess are of substantially equal and uniform diameters.

19. The holder of claim 1 wherein said liner is of uniform outer diameter throughout its length.

20. The holder of claim 1 including a seal element surrounding said plunger and disposed adjacent said end of said support member.

21. The holder of claim 20 wherein said liner recess has an enlarged portion adjacent said end of said support member to contain said seal element.

22. The holder of claim 1 wherein said plunger is of uniform diameter throughout its length.

23. The holder of claim 1 wherein said plunger is slidably fitted within the bore of said support member.

24. An expansible work holder comprising:

a tubular radially deflectable shell having both ends closed wherein one end has a longitudinal bore therethrough,

an expansible liner closely fitted within said shell and having a fluid-filled chamber therein, and

means advancing along said bore and selectively displacing a portion of said liner inwardly to increase the fluid pressure in said chamber for acting expansively on said liner and consequently on said shell.

25. An expansible work holder comprising:

a longitudinal body having a tubular chamber intermediate its ends to form a peripheral radially deflectable thin wall and a longitudinal bore in each end of said body opening into said chamber,

an expansible liner closely fitted within said chamber and having a fluid-filled recess opening to and registering with said bores,

a plunger extending from each bore into said recess,

and

means for separately selectively actuating said plungers to vary the degrees of extension thereof into said recess and thereby vary the fluid pressure therein acting expansively on said liner and consequently on said thin wall.

26. The holder of claim 25 wherein said plungers fit closely on their diameters within said liner recess.

27. The holder of claim 25 wherein said bores each have a threaded outer end and said actuating means comprises a set screw carried in each outer end and bearing on the end of the respective plunger in its bore.

28. The holder of claim 25 and including a seal element surrounding each plunger adjacent the opening of said recess to the respective bore for sealing against fluid leakage from said recess.

I I I 

1. An expansible work holder comprising: a support member having a longitudinal bore open to an end of said member, a tubular radially deflectable shell extending from said end of said support member and having a closed end to define a chamber wholly within said shell between its closed end and said end of said support member, an expansible liner closely fitted within and substantially filling said chamber and having a fluid-filled recess opening to and registering with said bore at said end of said support member, a plunger extending from said bore into said recess, and means selectively actuating said plunger to vary the degree of extension thereof into said recess and thereby vary the fluid pressure therein acting expansively on said liner and consequently on said shell.
 2. The holder of claim 1 wherein said plunger fits closely on its diameter within said liner recess.
 3. The holder of claim 1 wherein said bore has a threaded outer end and said actuating means comprises a set screw adjustably carried in said outer end and bearing on the end of said plunger in said bore.
 4. The holder of claim 1 and including a seal element surrounding said plunger adjacent the opening of said recess to said bore for sealing against fluid leakage from said recess.
 5. The holder of claim 1 wherein the closed end of said shell has a longitudinal bore therethrough and the recess in said liner extends therethrough tO be open to and register with said shell bore, and including a second plunger extending from said shell bore into said recess, and means selectively adjusting said second plunger to vary the degree of extension thereof into said recess and thereby initially adjust the fluid pressure therein which may be subsequently varied by actuation of said first plunger.
 6. The holder of claim 5 wherein said second plunger fits closely on its diameter within said liner recess.
 7. The holder of claim 5 wherein said shell bore has a threaded outer end and said adjusting means comprises a set screw adjustably carried in said shell bore outer end and bearing on the end of said second plunger in said shell bore.
 8. The holder of claim 5 and including a seal element surrounding said second plunger adjacent the opening of said recess to said shell bore for sealing against fluid leakage from said recess.
 9. The holder of claim 1 wherein said plunger has a bore therethrough open to said recess and a second plunger longitudinally movable therein to adjust fluid pressure in said recess.
 10. The holder of claim 9 and including a seal element surrounding said second plunger and sealing same within said first plunger bore against fluid leakage from said recess.
 11. The holder of claim 5 wherein said second plunger has a bore therethrough provided with a one-way valve to provide for selectively filling said recess with fluid.
 12. The holder of claim 1 wherein said shell includes a portion of a different size than the remainder of said shell for providing a two-line contact with a bore within a workpiece upon expansion of said holder and consequently said shell.
 13. The holder of claim 12 wherein said portion of a different diameter comprises a slightly smaller diameter region of said tubular shell.
 14. The holder of claim 12 wherein said portion of a different diameter comprises a pair of longitudinally spaced annular lands.
 15. The holder of claim 1 and including means selectively displacing a portion of said liner inwardly to reduce the volume of said recess for adjusting the initial fluid pressure therein which may be subsequently varied by actuation of said plunger.
 16. The holder of claim 1 wherein said shell is separable from and removably secured to said support member.
 17. The holder of claim 1 wherein said liner recess is of uniform diameter throughout its length.
 18. The holder of claim 1 wherein said tubular member bore from which the plunger extends and said liner recess are of substantially equal and uniform diameters.
 19. The holder of claim 1 wherein said liner is of uniform outer diameter throughout its length.
 20. The holder of claim 1 including a seal element surrounding said plunger and disposed adjacent said end of said support member.
 21. The holder of claim 20 wherein said liner recess has an enlarged portion adjacent said end of said support member to contain said seal element.
 22. The holder of claim 1 wherein said plunger is of uniform diameter throughout its length.
 23. The holder of claim 1 wherein said plunger is slidably fitted within the bore of said support member.
 24. An expansible work holder comprising: a tubular radially deflectable shell having both ends closed wherein one end has a longitudinal bore therethrough, an expansible liner closely fitted within said shell and having a fluid-filled chamber therein, and means advancing along said bore and selectively displacing a portion of said liner inwardly to increase the fluid pressure in said chamber for acting expansively on said liner and consequently on said shell.
 25. An expansible work holder comprising: a longitudinal body having a tubular chamber intermediate its ends to form a peripheral radially deflectable thin wall and a longitudinal bore in each end of said body opening into said chamber, an expansible liner closely fitted within said chamber and having a fluid-filled recess opening to and registering with said bores, A plunger extending from each bore into said recess, and means for separately selectively actuating said plungers to vary the degrees of extension thereof into said recess and thereby vary the fluid pressure therein acting expansively on said liner and consequently on said thin wall.
 26. The holder of claim 25 wherein said plungers fit closely on their diameters within said liner recess.
 27. The holder of claim 25 wherein said bores each have a threaded outer end and said actuating means comprises a set screw carried in each outer end and bearing on the end of the respective plunger in its bore.
 28. The holder of claim 25 and including a seal element surrounding each plunger adjacent the opening of said recess to the respective bore for sealing against fluid leakage from said recess. 